In an icy and snowy road pneumatic tire, as improving measures for on-ice performances, increasing an edge amount of sipes provided in a block thereof, reducing hardness of tread rubber, and the like have been generally attempted. However, in the case where tread rubber is reduced in hardness, block stiffness is reduced. This causes the block to fall down during braking and driving and during cornering, and hence leads to reduction in ground contact area of the tire, and as a result, tire performances both during summer and during winter are deteriorated. Accordingly, for the purpose of preventing the block from falling down, it has been proposed that a sipe should be formed in a three-dimensional shape.
As a sipe having a three-dimensional shape, one where a zigzag shape is formed on a tread surface, and an amplitude of the zigzag shape is varied inside the block has been proposed (for example, refer to Patent Document 1). In this case, although it is possible to increase block stiffness during braking and driving, there is a disadvantage that an effect of increasing block stiffness during cornering can be scarcely obtained.
In addition, a sipe where a zigzag shape is formed on a tread surface, and triangular pyramids and inverted triangular pyramids are alternately arranged inside the block has been proposed (for example, refer to Patent Document 2). In this case, although it is possible to expect an effect of increasing block stiffness during cornering, there is a disadvantage that a place where it can be arranged is limited because the sipe has a direction.
Moreover, a sipe where a zigzag shape is formed on a tread surface, and bent portions ranging in a tire radial direction and being bent in a tire widthwise direction are formed inside the block, has been proposed (for example, refer to Patent Document 3). In this case as well, there is a disadvantage that block stiffness during cornering is low as compared to block stiffness during braking and driving.
[Patent Document 1] Japanese patent application Kokai publication No. 2000-6619
[Patent Document 2] Japanese patent application Kokai publication No. 2002-301910
[Patent Document 3] Japanese patent application Kokai publication No. 2002-321509